1. Field of the Invention
The present invention relates to construction machinery such as a hydraulic shovel.
2. Description of the Related Art
Conventional construction machinery is mainly of a hydraulic driving type. In a hydraulic shovel, for example, hydraulic actuators (hydraulic cylinder, hydraulic motor) allow a working machine to drive, a revolving superstructure to turn, and a lower traveling structure to travel. In the shovel, pressure oil is discharged from a hydraulic pump driven by the engine, and the pressure oil supplied to these hydraulic actuators is controlled, so that works are performed.
Works of an hydraulic shovel do not always need to exhibit 100% of the capacity with reference to the capacity of the engine. Instead, there are many works which only need to exhibit 90% or 80% of the capacity. That is to say, as shown in the engine torque characteristic diagram shown in FIG. 4, there set working modes such as a point PS showing a “usual-load mode” for performing works with usual load, and a point PL showing a “light-load mode” for performing works with light load, with reference to a point PH showing a “100% load mode” for performing load works with 100% output. At each of the points PH, PS and PL, an iso-horsepower control is performed so as to match the driving torque of the hydraulic pump to the output torque of the engine (discharge amount of the hydraulic pump is controlled according to PQ curve (iso-horsepower curve) so as to gain the output torque of the matching point), to thereby efficiently utilize the engine output and improve the fuel consumption. Note that the driving torque of the hydraulic pump means a torque which is requested from the hydraulic pump to the engine in order to drive hydraulic actuators.
A hydraulic shovel mounts an engine having an output which coincides with the largest necessary horsepower in a case that a vehicle performs a work, that is, such an engine that the rating output point PH of the engine torque curve coincides with the largest necessary horsepower curve L shown in FIG. 4. Recently, there are demands for miniaturizing the engine so as to increase fuel efficiency. In this case, when the engine is simply miniaturized, there may arise a case that the necessary driving torque is not achieved in the hydraulic pump even when the capacity of the engine is 100% used. Therefore, restrictions (provisions) are provided with respect to the miniaturization of engines so that the miniaturization has not aggressively taken place.
In order to solve these problems, a hybrid-type construction machinery including, for example, an engine, a power generator driven by the engine, a battery which charges electric power generated by the power generator, and an electric motor driven by the electric power of the battery, has been proposed conventionally (See, for example, the Japanese Utility Model Laid-open No. 5-48501).
FIG. 5 shows the relation between the number of revolutions of the engine and the engine torque. In FIG. 5, curves 1 to 7 show iso fuel-consumption efficiency (iso fuel-consumption rate), respectively, in which the fuel consumption efficiency is improved from the curve 1 to the curve 7. Here, the fuel-consumption efficiency means a necessary fuel weight per horsepower (g/HP). A curve 50 shows the limit curve of the controllable range of a governor controlling the engine speed. The range outside of the curve 50 shown by the virtual curves does not exist actually.
Now, the operational control of the hybrid-type construction machinery will be explained referring to FIG. 5. In the case of a light-load mode (light-load state) where the arm, the bucket and the like oscillate, the number of revolutions of the engine and the engine torque are small, shown, for example, as the point A (torque T1). In this case, the fuel-consumption efficiency is represented as the curve 1. In the light-load mode, the engine torque is increased while the number of revolutions of the engine remains as it is, so as to generate, for example, the torque T2 equivalent to the point B on the curve 2. The difference between the T1 and T2 is used as the surplus torque for generating power in the power generator, to thereby charge the battery.
In the case of a high-load mode (high-load state) where the necessary driving torque in the hydraulic pump is larger than that of the rating output point of the engine, that is, in the case that the driving torque needs the torque T4 at the point D, it is accommodated by adding the torque (T4−T3) to the torque T3 by the engine at the point C. The fuel-consumption efficiency of this case exhibits the highest fuel-consumption rate in the curve 7.
In the conventional light-load mode described above, although the fuel-consumption efficiency is slightly improved, it is far from the best point. That is to say, it may not be the operation in which the fuel-consumption efficiency is considered. Further, in the high-load mode, the engine does not run at a point approximate to the rating output point where nearly 100% capacity of the engine is to be exhibited. Therefore, in order to output the necessary driving torque, a larger torque should be added (assisting torque). Consequently, the electric motor needs larger output, which may result in that the torque is not achieved. Moreover, the output of the engine has not been effectively utilized.